1. Field of the Invention
This invention pertains to signal processing circuits for the dual sampling of correlated signals produced by image sensing devices, particularly charge-transfer devices.
2. Description Relative to the Prior Art
A signal processing technique known as correlated double sampling is commonly used to process the output signal from a charge-coupled device (CCD) image sensor in order to obtain low noise performance. Correlated double sampling is equivalent to subtracting a reset reference level (V.sub.reset) from an image level (V.sub.image) for each output pixel from the CCD image sensor.
A CCD output circuit, shown in FIG. 1, converts the photoelectrically generated signal charge to a voltage signal. Charge packets from the CCD imager photosites (not shown) are transferred into a horizontal shift register 10. The charge packets are shifted horizontally via horizontal clocks H1 and H2 and onto a floating diffusion output 12 via output gate 13. The potential of the floating diffusion 12 changes linearly in proportion to the number of electrons in the charge packet. The potential of the floating diffusion 12 is applied to the input gate of a two stage source follower circuit 14, producing a signal at V.sub.out. A reset transistor 16 driven by the reset clock RESET recharges the floating diffusion 12 to the positive potential V.sub.rd before the arrival of each new charge packet from the horizontal shift register 10.
FIG. 2(A) shows the signal waveform V.sub.out at the output of the source follower 14. The waveform contains three components: the reset clock feedthrough (V.sub.ft), the reset reference level (V.sub.reset), and the image level (V.sub.image). The feed through V.sub.ft occurs as a result of capacitive coupling between the RESET gate 16 and the floating diffusion 12. When the floating diffusion 12 is reset, the exact reset voltage is affected by "thermal" noise, whose level depends on the capacitance of the floating diffusion 12 and the temperature. The same random reset noise voltage affects the level of both the reference level V.sub.reset and the image level V.sub.image. By taking the difference between samples of V.sub.reset and V.sub.image for each pixel, this "thermal" noise can be eliminated. This also reduces low frequency noise from the two stage source follower output amplifier 14.
A commonly known circuit for performing correlated double sampling is shown in FIG. 3 (see, for example, the circuits disclosed in U.S. Pat. Nos. 4,283,742 and 4,845,382). The signal V.sub.out from the circuit of FIG. 1 forms the input signal V.sub.in to sample/hold circuits 20 and 22, and the output of the sample/hold circuit 20 is further sampled by a sample/hold circuit 24. The aforementioned difference signal V.sub.D is taken between the outputs of the sample/hold circuits 22 and 24 by a subtracting circuit 26. FIGS. 2(B) and 2(C) show the sampling waveforms S/H(1) and S/H(2) that respectively drive the sample/hold circuit 20, and the sample/hold circuits 22 and 24. Sampling pulses from the waveform S/H(1) cause the sample/hold circuit 20 to sample the reset reference level (V.sub.reset). Sampling pulses from the waveform S/H(2) cause the sample/hold circuit 22 to sample the image level (V.sub.image), while simultaneously causing the sample/hold 24 to sample the output of the sample/hold circuit 20, thus effecting a delay in the reset reference level (V.sub.reset). A noise-free image signal V.sub.D (shown in FIG. 2(D)) is then obtained from the differencing circuit 26 by taking the difference between the outputs of the sample/hold circuits 22 and 24.